The long-term goal of this project is to understand how genetic alterations of the NF-(B2 gene may lead to tumorigenesis. The NF-kappaBeta2 gene encodes a protein of approximately 100-kDa (p100) that can promote apoptosis via its C-terminal death domain and/or lkappaB activity. In response to certain signals, p100 is processed to generate a 52-kDa transcriptional factor (p52) corresponding to the N-terminal half of p100. Aberrant activation of p52 has been observed in breast cancers. Also, C-terminal deletions and rearrangements of the NF-kappaB2 gene occur recurrently in a variety of B- and T-cell lymphomas. A fundamental feature of these genetic alterations is the generation of C-terminally truncated NF-kappaB2 mutants with oncogenic potential and the inactivation of p100 as an apoptotic protein. Our recent work suggests that p100 is a crucial regulator of TNF-alpha- and activation-induced apoptosis in thymocytes, and an inhibitor of the oncogenic activity of the tumor-derived NF-kappaB2 mutant p80HT in cells and in animals. These findings lead us to hypothesize that p100 is a tumor suppressor that promotes apoptosis and acts as a built-in defense against oncogenic mutations of NF-kappaB2 and aberrant activation of p52. We will test this hypothesis in both cell- and animal-based systems. In animal-based studies (Aim 1), we will establish p100 as a tumor suppressor against lymphomagenesis in transgenic mice with targeted expression of p80HT or p52 in lymphocytes; we will examine the possibility that the NF-kappaB2 p100 gene is haplo-insufficient for tumor suppression in these mouse models; the potential of p100 as a general tumor suppressor will be assessed in E/mu-myc transgenic mice. In cell-based studies (Aim 2), we will use bone marrow cell transformation as readout to define the biochemical activity of p100 essential for its anti-oncogenic function and the apoptotic pathway through which p100 suppresses oncogenic transformation; we will characterize apoptotic responses in lymphocytes and lymphoma cells from the animals with defined NF-kappaB2 mutations to identify at molecular levels the defects in apoptosis regulation that are linked to lymphomagenesis; we will use thymic activation-induced apoptosis as a model to delineate the molecular mechanism for p100 as an apoptotic protein. These studies may define a new class of tumor suppressors that act as a surveillance mechanism against oncogenic mutations of their own genes.